1. Field of the Invention
This invention relates to a method for controlling the by-pass air flow on deceleration of internal combustion engines, and more particularly to a method for controlling the by-pass air flow to prevent temporary over-enrichment of the air-fuel ratio in an initial stage of deceleration.
2. Description of the Prior Art
For preventing the air-fuel ratio from becoming overich on deceleration of an internal combustion engine, there has been known an apparatus (of the so-called dashpot) which employs a vacuum diaphragm thereby to stop the throttle valve once in an open position of a certain angle and then close same gradually to the fully closed position to prevent a sudden shutoff of the throttle valve.
With the above-mentioned apparatus, the throttle valve is slowly moved into the fully closed position from the predetermined open angle position, but the throttle opening is abruptly reduced from a wide-open position until the predetermined open angle position is reached. Accordingly, when the throttle is returned under a high engine speed condition, the torque difference is increased by the abrupt reduction of the throttle valve opening, causing a shock in the initial stage of deceleration and over-enriching the air-fuel ratio due to an abrupt drop of the absolute pressure in the intake pipe downstream of the throttle valve.
The characteristics of the above-mentioned dashpot are shown in FIG. 6, a correlation diagram of the number of engine revolutions Ne versus the intake pipe absolute pressure PBA, in which unloaded characteristics of the engine are indicated by the rightwarly rising curve I. Especially, the point Q at the terminal end of this characteristics curve I indicates the idling condition. The engine characteristics under the fully closed throttle condition (idling throttle opening) are indicated by the hyperbola II, while the engine characteristics under the conditions where the throttle valve is in the predetermined open angle position of the aforementioned dashpot are indicated by the curve III (one-dot chain line). Namely, when the accelerator pedal is depressed and then released, the conventional dashpot operates to drop the intake pipe absolute pressure onto the curve III from a point above the curve I indicative of an engine speed Ne higher than the idling speed and then gradually to the fully closed throttle curve II toward the point Q. However, since there is a large difference in the intake pipe absolute pressure PBA between the curves I and III at high engine speeds as indicated by a broken line, the air-fuel ratio tends to become overich in most cases, causing a large variation in torque in the initial stage of deceleration.